Invasion of Pieris mannii butterflies across Central Europe facilitated by urbanization

Urbanization progresses world-wide and the frequency of biological invasions increases. Understanding to what extent urban environments facilitate biological invasions and how this affects ecosystems within and outside urbanized areas thus becomes crucial. We here examine the role of urban environments in the invasion of the butterfly Pieris mannii that expanded across Central Europe within the last two decades. Using standardized butterfly captures at paired urban and (semi)natural field sites within the invaded range in Switzerland, we demonstrate that P. mannii is strongly associated with cities and towns. At least in some urban localities, this species is now the most common butterfly, and densities tend to be particularly high where urban areas are large. Because urban habitats commonly provide non-native host plants and perhaps also the physical structure and microclimate suitable to this butterfly, urbanization has clearly promoted the species' rapid invasion. Studying phenology over an entire season, we further infer that P. mannii has up to six generations per year, which may allow the species to adjust its life cycle to changing season length during northward expansion. Overall, our study demonstrates how preadaptation to urbanized environments in a relatively specialized insect increases urban biodiversity on a large geographic scale.